The present invention relates generally to motors comprising one or more electroactive polymers. More particularly, the present invention relates to clutch motors and their use in various applications.
A motor converts from an input energy to mechanical energy. Most often, the mechanical energy is output as rotary motion of a shaft, but linear motors for translating a shaft are also commonly used. The most common class of input energy for a motor is electricity. Electric motors include AC, DC, servo, and stepper motors. Compressed air and pressurized hydraulic fluid are also used to power air and hydraulic motors. Gasoline or diesel engines are another traditional class of motors that rely on combustion of a fuel. Each of these motor classes has advantages and disadvantages that influence their usage.
For a DC motor, DC current is typically supplied from battery sources. Battery voltages are typically in multiples of 1.5 volts, with 12 volts being the most common. DC motors are made in different electrical configurations, each of which provides a different torque-speed relationship that describes how the motor will respond to an applied load at different speeds. For a permanent magnet DC motor, torque often varies inversely with speed. Since the power available for a DC motor is typically limited, an increase in torque requires a decrease in velocity and vice versa. Thus, when a load is applied, the motor must reduce speed to compensate. One solution to the torque-speed problem is to use a ‘speed-controlled DC motor’, which contains a controller that increases and decreases current to the motor in the face of changing load to try and maintain a constant speed. These motors are typically expensive and run from an AC source since the controller converts from AC to DC.
AC motors provide continuous rotary motion but usually rely on current supplied by power companies. They are limited to a few speeds that are a function of the AC line frequency, e.g., 60 Hz in the U.S. The most common AC motor no-load speeds are 1725 and 3450 revolutions per minute (rpm), which represent some slippage from the more expensive synchronous AC motors speeds of 1800 and 3600 rpm. If other outputs speeds are desired, a gearbox speed reducer is attached to the motor's output shaft.
AC and DC motors are designed to provide continuous rotary output. Though they can be stalled against a load, they will not tolerate a full voltage, zero velocity stall for an extended period of time without overheating.
Servomotors are fast response, closed loop control motors capable of providing a programmed function of acceleration or velocity, or capable of holding a fixed position against a load. Thus, precise positioning of the output device is possible, as is control of the speed and shape of its time response to changes in load or input commands. However, these devices are very expensive and are commonly used in applications that justify their cost such as moving the flight control services of aircraft.
Stepper motors are designed to position an output device. Unlike servomotors, these are typically open loop, meaning they receive no feedback as to whether the output device has responded as requested. While being relatively good at holding the output in one position for indefinite period, they often are poor with motion and get out of phase with a desired control. In addition, these motors are moderately expensive, have a low torque capacity, and also require special controllers.
Most electromagnetic motors must consume electrical energy to maintain a force or torque. The only exceptions would be motors with preferred magnetic positions such as stepper motors that can resist a torque up to the torque that causes rotor slippage. But even stepper motors cannot provide a constant static torque at an arbitrary rotor position unless power is used. Thus, conventional electromagnetic motors typically use power even to hold a static torque where no external work is done. This is why at stall and low speed conditions the efficiency of almost all electromagnetic motors is poor.
Air and hydraulic motors have more limited application than electric motors since they require the availability of a compressed air or hydraulic source. Both these classes of motors provide poor energy efficiency due to the losses associated with the conversion of energy first from chemical or electrical energy to fluid pressure and then to mechanical output. Although individual air motors and air cylinders are relatively cheap, these pneumatic systems are quite expensive when the cost of all the ancillary equipment is considered.
In addition to the specific drawbacks discussed with respect to each class of motor, all of the above motors classes are generally heavy, bulky and not suitable for many applications such as those requiring light weight continuous output. In view of the foregoing, improved systems that convert from an input energy to mechanical energy would be desirable.